For example, in a radar oscillator for use in a transmitter section for transmitting an oscillation signal of low power such as a UWB radar as a short distance radar device for car mounting or for a blind person, a medical application and the like, an output of an oscillation signal having a quasi-millimeter wave (22 to 29 GHz) is intermittently continued by a pulse signal indicating a transmission timing of a radar wave from the outside section.
FIG. 11 is a block diagram depicting a circuit configuration of a conventional radar oscillator 10 of such a type.
That is, in the radar oscillator 10, an oscillating unit 11 has an amplifier 12, a resonator 13 connected to an output section of the amplifier 12, and a feedback circuit 14 which positively feeds back an output of the amplifier 12 to an input side, thereby oscillating a signal of a frequency which depends on the resonator 13.
An oscillation signal output from the oscillating unit 11 is input to a switch 15 (an electronic switch such as a semiconductor) which is periodically opened and closed by a pulse signal P indicating a transmission timing of a radar wave.
Then, when the pulse signal P is at a first level (for example, low level), the switch 15 is closed, and an oscillation signal S is output. When the pulse signal is at a second level (for example, high level), the switch 15 is opened, and the oscillation signal S is not output.
However, in the conventional radar oscillator 10 which periodically opens and closes an output passage of the oscillation signal by the switch 15 as described above, the oscillating unit 11 itself of the radar oscillator 10 is always in an operating state (oscillating state) regardless of the opening and closing of the switch 15 while the switch 15 is opened when the pulse signal P is at the second level (for example, high level). Thus, even while the switch 15 is opened, the oscillation signal from the oscillating unit 11 leaks through an equivalent high frequency stray capacitance component, a high frequency parasitic capacitance component or the like of the switch 15. Therefore, there is a problem that the oscillation signal output cannot be stopped completely.
In particular, as described previously, it is difficult to prevent a leak from the switch 15 at a high frequency bandwidth of 22 to 29 GHz.
FIGS. 12A and 12B are timing charts each showing an operation of the above-described conventionally configured radar oscillator.
That is, although an oscillation signal S as shown in FIG. 12B is output during a low level period of a pulse signal P shown in FIG. 12A, a leak component S′ of the oscillation signal is output during a high level period of the pulse signal. Thus, an output ratio between the low level period and the high level period is not obtained as only about 20 dB.
The leak component S′ restricts a substantial receiving sensitivity of a reflection wave with respect to a radar wave output at a regular transmission timing, thus narrowing a radar search range and making it difficult to detect an obstacle of a low reflection index.
In addition, with respect to the above-described UWB radar system, the Federal Communication Committee (FCC) restricts that the average power density in a bandwidth of 22 to 29 GHz be −41 dBM/MHz or less and the peak power density be 0 dBM/50 MHz or less in Non-Patent Document 1 below.
Non-patent document 1: FCC02-48, New Part 15 Rules, “FIRST REPORT AND ORDER”
Namely, in the above UWB radar system, the total amount of energy in the bandwidth of 22 to 29 GHz is restricted. Thus, if the leak component S′ is large, the output level of a regular oscillation signal must be set low concurrently, and the search distance or the like is largely restricted.